Lumber which has recently been cut contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried. Drying removes a large amount of water from the lumber and significantly reduces the potential for the lumber to become warped or cracked. Acceptable water content varies depending on the use of the lumber and type of wood; however, a moisture content of about nineteen percent, or less, is acceptable in many circumstances.
Although lumber may be dried in the ambient air, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. The charge of lumber typically consists of two or more rectangular stacks of lumber. A typical kiln chamber is a generally rectangular building that can be at least partially sealed to control the amount of air that is introduced to and exhausted from the kiln chamber. Hot air from a furnace is forced through an inlet duct to a plenum that is positioned in an upper portion of the kiln chamber, and the hot air is discharged from the plenum to the chamber through multiple openings defined in the top of the plenum. The heated air supplied to the chamber is circulated within the chamber by fans so that the heated air flows along a flow path that extends through one or more upstream stacks of lumber, and thereafter through one or more downstream stacks of lumber.
Hot air from the plenum also flows into and escapes from a row of reheaters. It is conventional for each of the reheaters to be a downwardly extending pipe-like structure that extends from the bottom of the plenum, is positioned between the upstream and downstream stacks of the lumber, and extends into the portion of the flow path that is between the upstream and downstream stacks of the lumber. In some kilns, each of the reheaters is rectangular in a horizontal section and defines elongate vertical slits through which hot air flows into the chamber. The hot air discharged from the reheaters serves to further heat the air circulating through the kiln, thereby at least somewhat compensating for the heat that has been lost in drying the upstream stack of lumber prior to introducing the reheated air into the downstream stack of lumber. Unfortunately, the reheaters contribute to the resistance to flow along the flow path since they extend into the flow path.
Each stack of lumber being dried also contributes to the resistance to flow along the flow path. As best understood with reference to FIGS. 16-17, it is conventional for each stack 248 of a charge of lumber to consist of a number of vertically stacked, horizontal rows 250 of lumber 252 that are arranged such that cross-sections of the stack are generally rectangular. The horizontal rows 250 are spaced apart with narrow wooden boards 254, or the like, referred to as "stickers." The stickers 254 are positioned between each horizontal row 250 to space the rows apart so that multiple passages 256 are defined between adjacent layers and are open at the opposite sides of the stack 248. The heated air traveling along the flow path passes through the passages 256 and is in direct contact with both the upper and lower surfaces of individual pieces of lumber 252 so that the lumber is dried. For each of the passages 256, airflow therethrough is such that layers of viscous air are developed proximate to the surfaces of the pieces of lumber that face and define the passage. Those viscous layers are referred to as boundary layers 260. The boundary layers 260, which are areas of retarded flow, are caused by the viscous interaction between the airflow through the passage 256 and the surfaces of the pieces of lumber 252 that define the passage, as well as interaction between the airflow and the lumber surfaces that are proximate to the inlet opening of the passage.
Each boundary layer 260 includes an initially protruding portion 262 (i.e., a separated region) at the entrance of its passage 256. The protruding portion 262 tapers to a generally planar portion 264. For each of the boundary layers 260, the protruding portion 262 is a portion of the boundary layer that has become separated from the surface or surfaces of the one or more pieces of lumber 252 that define the passage. The separation occurs because of interaction between the airflow and an edge or edges of the one or more pieces of lumber 252 that define the inlet to the passage.
It is conventional for the edges of the layers of lumber 252 to be aligned so that they generally extend in a common plane. As a result, for each of the passages 256, the protruding portions 262 of the boundary layers 260 are aligned in a manner that is very restrictive to flow, since the boundary layers are regions of retarded flow and thereby tend to block flow into the passage. More specifically, an unrestricted flow path exists only in that region between the boundary layers 260 of each of the passages 256. Those unrestricted flow paths are characterized by generally fully developed two dimensional channel flow. Within each passage 256, the protruding portions 262 are aligned in a manner that causes a significant reduction in the size of the unrestricted flow path at the entrance of the passage. It is generally characterized as a poor entrance, similar to a flanged pipe condition but for a two dimensional channel.
The resistance to flow along the flow path that is caused by the reheaters and the stacks of lumber reduces the speed at which the pieces of lumber can be dried, which can be disadvantageous since mill production depends upon the ability to dry lumber at a sufficient rate so that production need not be slowed to allow for the drying process. The resistance to flow along the flow path that is caused by the reheaters and the stacks of lumber also requires significant pressure increases to maintain the flowrate; therefore, the kiln fans, which force the heated air to flow along the flow path, must work excessively, which is disadvantageous. Operating the fans of a kiln consumes energy that adds to the cost of producing quality lumber. Of course it is advantageous to lower the cost of producing quality lumber. Whereas some conventional kilns can be characterized as being efficiently operated and able to dry lumber at a sufficient rate, there is always a demand for new kilns and kiln-related structures that can be even more efficiently operated, and that facilitate the drying of lumber at a sufficient rate.